A ground fault circuit interrupter is a small device that watches the balance between the current going out on the hot conductor and the current coming back on the neutral. In a healthy circuit those two match. When they do not, current is escaping somewhere, and the most common somewhere is through a person.
The device reacts to a difference of a few milliamps, and it reacts in a fraction of a second. That is fast enough and sensitive enough to stop a shock from becoming a fatal one. This is why these outlets are required in wet locations, where the path to ground through a person is far easier.
The standard locations are bathrooms, kitchens, garages, unfinished basements, crawl spaces, outdoors, and anywhere near a sink. The exact requirements have expanded over the years, which means an older home that was compliant when built may have far fewer of these than a new one.
Testing them is simple and worth doing twice a year. Press the test button. The reset button should pop out and the outlet should go dead. Press reset and power returns. If pressing test does nothing, the device has failed and should be replaced. A failed device that still passes power provides no protection at all, and there is no way to tell from the outside.
One point that confuses people is downstream protection. A single device can protect other ordinary outlets wired after it on the same circuit. Those downstream outlets often carry a sticker saying they are protected. This is legitimate and common, but it means the test button for a bathroom outlet might physically be in the garage. Worth knowing before you go hunting for a tripped device.
Nuisance tripping happens. Older motors, some appliances with heating elements, and long circuit runs with damp conditions can cause it. Before assuming the device is faulty, unplug everything on the circuit and see whether it holds.
These devices have a service life. Most manufacturers suggest replacing them after ten to fifteen years even if they still appear to work. They are inexpensive, and the failure mode is silent.
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